Many problems in application of maxima and minima may be solved easily by making use of trigonometric functions. The basic idea is the same; identify the constant terms and identify the variable to be maximized or minimized, differentiate that variable then equate to zero.
Problem 72
A light is to be placed above the center of a circular area of radius a. What height gives the best illumination on a circular walk surrounding the area? (When light from a point source strikes a surface obliquely, the intensity of illumination is
$I = \dfrac{k \sin \theta}{d^2}$
where θ is the angle of incidence and d the distance from the source.)